Some Fixes to the UI behaviour

Moved the delete_event signal handling to the ContactListWindow init_signals()
function (together with the one handling the minimize event) as it makes more sense.
Fixed 2 bugs:
1-Now changing the "minimize on tray" option actually change the behaviour
of the app when the user press the minimize button
(no longer forcing the user to kill and relaunch the application);
2-Fixed the window staying hid when minimized with the "minimize on tray"
 option active